Device for measuring mach number



Aug. 15, 1961 E. J. HARDGRAVE, .JR

DEVICE FOR MEASURING MACH NUMBER Filed June 24. 1953 ATTORNEYS United States Patenti O 2,995,932 DEVICE FOR MEASUR'ING MACH NUMBER Everett Jt Hardgrave, Jr., Silver Spring, Md., assgnor to the United States of America as represented by the Secretary of the Navy Filed June 24, 1953, Ser. No. 363,783 '5 Claims. (Cl. 73--212) This invention relates to a measuring device and more particularly to a device for measuring theMach number of a supersonic airstream. Inasmuch as the `same flow conditions arise for a body passing through the air at supersonic speeds as for a body stationary with respect to the earth but being subjected to an airstream owing at supersonic speeds, the term supersonic airstream shall, for the purposes of this invention, include both situations.

The mathematical relationships of static pressure, total pressure, density, speed etc. for low speed flow are based upon the valid assumptions of incompressible tlow theory. However, high speed tlow is characterized by compressibility such that the mathematical relationships depicting low speed ow do not apply. Moreover, the total pressure of a supersonic airstream cannot be measured directly by the conventional Pitot tube,.and the commonly known static pressure tubes are less reliable for accurately determining the static pressure of the airstream. Therefore, the customary methods of' determining the speed of a subsonic airstream are ineffective for meas'- uring the speed of a supersonic airstream.

It is the principal object of this invention to provide a 'device for accurately measuring the Mach number of a supersonic airstream.

Another object of this invention is to provide a device capable of producing indications of pressure values from which the Mach number of an airstream may be' accurately determined.

Further objects and attendant advantages of this invention will become evident from the following detailed description, taken in conjunction with the appended drawing, in which:

FIG. l is an axial section of a device constituting this invention; and

FIG. 2 is a section on line 2-2 of FIG. 1.

In the exemplary embodiment of this invention as shown in FIGS. 1 and 2, a streamlined body 11 is comprised of four parts. The tirst of these parts, the dituser section 12, is conically formed, having an inlet 13 with a' sharp lip 14 at its convergent end 15, and a duct 16 deiined by the inner wall of said section. The duct 16 includes a short cylindrical portion 17 connecting the inlet 13 with a short diverging portion 18 which, in turn, communicates with a longer diverging portion 19 having a larger angle of divergence. 'Ihe duct 16 terminates in a cylindrical portion 21 open at the divergent end of the diiuser section 12. The divergent end portion 22 of the diffuser section 12 is reduced to define an external annular shoulder 23 and an externally threaded portion for a purpose to be described hereinafter.

The second part of the body 11 is the fore low velocity chamber section 24 which is cylindrically shaped, having a central duct 25. The forward end portion of this chamber section 24 is provided with an internal, annular recess 26 which is internally threaded to cooperate with the threaded divergent end portion 22 of the diffuser section 12. Pairs of opposed longitudinal grooves 27 are formed in the inner peripheral wall of the central duct 25 to accommodate four struts 28 joined at their inner ends by a sleeve 29. The divergent end portion 22 of the diffuser section 12, when connected to this chamber section 24, abuts the end portions of the struts 28 to retain them in position in the grooves 27.

ice

2 A tube 31 is mounted in the sleeve 29 and extends coaxially towards the inlet 13, said tube terminating in spaced relation to said inlet. In the forward end of the tube 31 is fixed a tapered chuck 32 having a central bore 33 for receiving a Pitot tube 34 having an open end 35: disposed in the plane of the inlet 13. The Pitot tubeL extends rearwardly through the tube 31 to emerge from' said tube at the approximate-position of the struts 28. From the tube 31 the Pitot tube 34 is bent ninety degrees and extends laterally through a suitable opening 36 in the fore chamber section 24. A pressure indicating' instrument 37, such as a gauge of lthe altimeter type or a manometer, is connected to the Pitot tube 34 to receive pressure sensations therefrom. The rear end portion of this chamber section 24 is reduced and externally threaded at 38 for connection to the third body part.

'Ihe third body part is the rear low velocity chamber section 39; formed with a cylindrical central duct 41 and an internal annular recess 42 and internal threads 43 on its forward end portion. In this case the recessl 42 extends inwardly beyond the threads 43 and receives a circular ow straightening screen 45 which traverses the duct 41. The reduced, threaded portion 38 of the chamber section 24 is screwed on the forward end portion of this section 39 and abuts the screen 45 to hold it in position. Y An opening 46 is provided in the wall of the chamber section 39 for accommodating a static pressure tube 47 perpendicular to the axis of said section. A pressure indicating instrument 4S is connected to the tube 47 to receive pressure sensations therefrom. The rear end portion of section 39 is reduced and externally threaded at 49. The rearmost body part comprises a sonic discharge nozzle 51 having an internal, annular recess 52 on its forward end portion which is internally threaded for attachment to the chamber section 39. v .v

In operation, the body'll is alined in the airstream to be investigated with the inlet 13 facing the oncoming air. The Pitot tube 34 senses the impact pressure Pt of the airstream and the 'instrument 37 provides an indication of this pressure. Air from the airstream enters the inlet 13 and passes into the duct 16 where it expands Vand decelerates to subsonic speeds in the diffuser portion 12. From the diifuser portion 12 the slowed -air ows into the chamber portions 24 and 39 past the liow straightening screen 45. The static pressure tube 47 senses the static pressure Pc in the rear chamber 39 and transmits said pressure to the instrument 48 for providing an indication. Inasmuch as the air flowing in the chamber portions 24 and 39 is at a low subsonic vspeedythe static pressure Pc, as sensed by the tube 47 and measured by the instrument 48, is an accurate representation of the true Static pressure in said chamber portions. The measured impact pressure Pt and chamber static pressure Pc may be related to the Mach number by equating the mass ow at the inlet 13 to that at the nozzle 51 as follows: t

(gpl/fl inlet: (gPVA )nozzle where:

p=density of the air V=velocity of ow A=crosssectional area of the airstream g=acceleration due to gravity Patented Aug. 15, 1961 and from the definition of velocity of ow, l/-QcM (c being the speed of sound), and the representation for the speed of sound c=\/R T, the velocity of ilow Imay be expressed in terms of Mach number and temperature:

Inasmuch las the total temperature of the air flowing into .the inlet 13 and through the nozzle 51 is constant the.l factor Tf1/2 maybe cancelled from the equation,

inlet' (APxiJnc/nzle In order to inject terms representing the measured impact pressure Pt and static pressure Pc into the equation the inlet side is multiplied by PHP( and the nozzle side by (5)52 .Pt ePo (Pt being the total pressure in the chamber and nozzle) and the terms rearranged as follows: Y

A Y{(P/Pt'nAP,.'1.1=[(i)(Qld/1arjan.

The term y (P/Pt's be expressed as a function of Mach number:

and (Pt/Pk is 1a function of the ratio of the cross-sectional area of the chamber to that of the nozzle and, therefore, is constant. Since iiow through a sonic discharge nozzle is at a Mach number equal to uni-ty the term on the nozzle side is constant. Therefore the mass ow balance equation can be rearrangedas follows:

wherein K designates a constant dependant upon the configuration and dimensions of the body 11 and the duct 16. The constant K may be computed directly from the known dimensions of the body 11 and the duct 16 or it may be found empirically by placing said body in an airstream of Iknown Mach number. i

The values of Y may be tabulated with. corresponding values of Mach number; a sample tabulation Ybeing shown below:

Table I (P/PKMI Mach number Upon measuring the values of the impact pressure Ph and static pressure Pc the term will be determined from which the Mach number will be found by reference to a tabulation similar to that shown in Table I.

It is within the scope of this invention to utilize the pressures Pt' and Pc in a suitable meter to directly produce Mach number indications in accordance with the relationship as expressed in the equation developed hereinabove.

Obviously many other modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

l. A device `for indicating the Mach number of a supersonic airstream, comprising means including a divergent rdiiuser for capturing a portion of the airstream and slowing it to a subsonc speed, a low velocity chamber connected to said capturing means for receiving said slowed air, screen means mounted in said chamber for straightening said slowed air, a convergent-divergent nozzle for sonically discharging said slowed air from said chamber, means including a Pitot tube for measuring the impact pressure of the airstream at the entrance of said diffuser, and means for measuring the static pressure within said chamber downstream of said screen means.

2. A device for indicating the Mach number of a supersonic airstream, comprising a divergent diffuser in an open-ended body having a duct with an inlet facing the airstream for capturing a portion of the airstream and slowing it to a subsonic speed, a convergent-divergent nozzle at the downstream end of said duct for sonically dscharging'said arstream, means including a screen located between said diffuser and nozzle for straightening the captured portion of said airstrearn, means for meas,- uring the impact pressure of said inlet, and means for measuring the static pressure within said duct downstream of said screen.

3. A device for indicating the Mach number of a 'supersonic airstream, comprising a body having a duct with an inlet facing fthe airstream, a diffuser section in said body for slowing the air entering the duct through said inlet, a chamber located downstream of said diffnser having screen means therein for straightening the slowed air from said diiuser, a Pitot tube mounted coaxially in said duct and having an open end disposed in the plane of said inlet facing the airstream, a static pressure tube opening into said duct downstream from said screen means, and an exit nozzle mounted on thc downstream end of said duct for sonically discharging said airstream. Y

4. A `device for indicating the Mach number of a supersonic airstream, comprising a body having an openended duct With an inlet for capturing a portion of said airstream, a divergent diiuserrsection connected to said inlet, a low velocity chamber connected to said diffuser section, a sonic discharge nozzle connected to said charn- 'ber for sonically discharging said airstream, a Pitot tube mounted coaxially in said body and having an open erid disposed in the plane of said inlet facing the airstrearn,

6 and a static pressure tube opening into said low velocity 1,426,698 Zahm Aug. 22, 1922 chamber. 2,343,282 Daiber Mar. 7, 1944 5. An arrangement as set forth in claim 4, With ad- 2,441,042 Stoll May 4, \1948 ditionally screening means mounted in said 10W velocity 2,482,701 Anderson Sept. 20, 1949 chamber for sn'aighcening the ow through said chamber. 5 2,570,129 Johnson Oct. 2, 1951 2,592,176 Orlin et al. Apr. 8, 1952 References Cited in the file of this patent 2,592,322 Nerod Apr. 8, \1952 2,618,972 Leduc Nov. 25, 1952 UNITED STATES PATENTS 2,641,105 Drake June 9, 1953 1,085,184 Weymouth Ian. 27, 1914 10 UNITED STATES PATENT 'OFFICE CERTIFICATE OE CORRECTION Patent N0., 2,995,932 August 15 1961 Everett J I-Iarolgrave,v Jr.,

It s hereby certified that error appears in the above numbered patsad Letters Patent should read as ent requiring correction and that the oor eoted below.

line I7 name of inventorS1 for "Everett J.

" inv line i4q same In the grant,

Hartgrave, Jr. rea lines 43 and 44, strike out, dvergen ended" and insert the same after "having a column 4I. i

Signed and sealed this 10th day of April 1962,

(SEAL) Attest:

ERNEST' 'w SWIDER DAVID L. LADD Commissioner of Patents Attesting Officer 

